Medical applications of microrobots driven inside blood vessels are numerous. Amongst these applications are Minimally Invasive Surgeries (MIS) like angioplasties, and highly localized drug deliveries for chemotherapy or biopsies. The smaller these microrobots are, the wider the operating range becomes through access to the finest blood vessels such as capillaries.
Because of its larger diameter, the arterial system of the human body is being considered as the initial target location where a microrobot can be implanted. With further miniaturization and more precise positioning techniques, smaller regions such as capillaries could also be considered.
The force for propelling and controlling displacement of the microrobot must be stronger than the drag force on this microrobot for motion to take place and to enable control of the displacement of the microrobot in the blood vessels. In order to determine the level of the force required for propelling and controlling displacement of the microrobot, the following parameters must be taken into consideration:
The dimensions of the blood vessels in human beings typically range from 25 mm in diameter (aorta) to approximately 8 μm (capillaries) [R. F. Rushmer, Structure and Function of the Cardiovascular System, W. B. Saunders Company, 1972].
The blood flow in the arterial system is pulsatile and much faster at the exit of the heart (ascending aorta: maximal systolic velocity is 1120 mm/s) [W. R. Milnor, Hemodynamics, Williams & Wilkins, 1982]. When an artery or vein bifurcates, the cross-sectional area of its branches exceeds that of the parent vessel. Therefore, the blood velocity decreases away from the heart in a similar fashion when water in a rushing stream slows down when entering a broad pool [R. F. Rushmer, Structure and Function of the Cardiovascular System, W. B. Saunders Company, 1972].
And, as described in [Wham, R. M., Basaran, O. A., and Byers, C. H., Wall effects on flow past solid spheres at finite Reynolds number, Industrial & Engineering Chemistry Research, Vol. 35, No. 3. March, pp. 864-874, 1996. 0888-5885. Oak Ridge Natl Lab, TN, USA], the drag force of a rigid sphere in a rigid cylindrical tube is a function of the density of the fluid (ρ), the velocity of the sphere relative to the velocity of the fluid (V), the frontal area (A), the drag coefficient of the sphere (CD), the Reynolds number (Re), and the ratio of the diameter of the sphere to the diameter (λ) of the cylindrical tube.
The use of micromotors for propelling a microrobot in blood vessels presents the following drawbacks. Micromotors require precise and complex assemblies of several moving parts having increased probability of failure and being difficult to miniaturize and fabricate [Morita, T., Kurosawa, M. K., and Higuchi, T., Cylindrical shaped micro ultrasonic motor utilizing PZT thin film (1.4 mm in diameter and 5.0 mm long stator transducer), Sensors and Actuators, A: Physical: The 10th International Conference on Solid-State Sensors and Actuators TRANSDUCERS '99, Jun. 7-Jun. 10, 1999, Vol. 83, No. 1. May, pp. 225-230, 2000. 0924-4247. Inst of Physical and Chemical Research (RIKEN), Saitama, Jpn]. Furthermore, micromotors need to carry along either an energy source or an energy conversion device (battery or induction coil for example), a device to convert the motor torque into motion force (propeller, flagella . . . ) and a device for controlling the direction of the driving force (flaps or MEMS nozzles). The smallest ultrasonic micromotors existing have a 1.4 mm diameter and are 5.0 mm long [Morita, T., Kurosawa, M. K., and Higuchi, T., Cylindrical micro-ultrasonic motor (stator transducer size: 1.4 mm in diameter and 5.0 mm long) Ultrasonics, Vol. 38, No. 1-8. March, pp. 33-36, 2000. 0041-624X. Inst of Physical and Chemical Research (RIKEN), Saitama, Jpn], and the propulsion group alone is already too big for vascular applications.
Magnetic propulsion for minimally invasive surgery researches has been implemented at University of Virginia [Grady, M. S., Howard, M. A. 3rd, Dacey, R. G. Jr, Blume, W., Lawson, M., Werp, P., and Ritter, R. C., Experimental study of the magnetic stereotaxis system for catheter manipulation within the brain, J Neurosurg, Vol. 93, pp. 282-8, August, 2000; 2] [Grady, M. S., Howard, M. A. 3rd, Broaddus, W. C., Molloy, J. A., Ritter, R. C., Quate, E. G., and Gillies, G. T., Magnetic stereotaxis: a technique to deliver stereotactic hyperthermia Neurosurgery, Vol. 27, pp. 1010-5; discussion 1015-6, December 1990.8] [McNeil, R. G., Ritter, R. C., Wang, B., Lawson, M. A., Gillies, G. T., Wika, K. G., Quate, E. G., Howard, M. A. 3rd, and Grady, M. S., Characteristics of an improved magnetic-implant guidance system, IEEE Trans Biomed Eng, Vol. 42, pp. 802-8, August, 1995] [McNeil, R. G., Ritter, R. C., Wang, B., Lawson, M. A., Gillies, G. T., Wika, K. G., Quate, E. G., Howard, M. A. 3rd, and Grady, M. S., Functional design features and initial performance characteristics of a magnetic-implant guidance system for stereotactic neurosurgery, IEEE Trans Biomed Eng, Vol. 42, pp. 793-801, August, 1995] and at Tohoku University [Ishiyama, K., Sendoh, M., and Arai, K. I., Magnetic micromachines for medical applications, Journal of Magnetism and Magnetic Materials, Vol. 242-245, No. 1. April, pp. 41-46, 2002. 0304-8853. Res. Inst. of Elec. Communication, Tohoku University, Aoba Sendai 980-8577, Japan] [Sato, F., Jojo, M., Matsuki, H., Sato, T., Sendoh, M., Ishiyama, K., and Arai, K. I., The operation of a magnetic micromachine for hyperthermia and its exothermic characteristic, 2002 International Magnetics Conference (Intermag 2002), Apr. 28-May 2, 2002, pp. 3362-3364, 2002] [Sendoh, M., Ishiyama, K., and Arai, K. I., Direction and individual control of magnetic micromachine, 2002 International Magnetics Conference (Intermag 2002), Apr. 28-May 2, 2002, pp. 3356-3358, 2002] [Sendoh, M., Ishiyama, K., Arai, K. I., Jojo, M., Sato, F., and Matsuki, H., Fabrication of magnetic micro-machine for local hyperthermia, 2002 IEEE International Magnetics Conference-2002 IEEE INTERMAG, Apr. 28-May 2, 2002, pp. FU11, 2002].
The concept studied at University of Virginia is the following: moving a ferromagnetic thermoseed (Video Tumor Fighter VTF) through brain tissue to reach a brain tumor. Once the tumor is penetrated, the thermoseed is heated via eddy current with RF excitation and is moved to scan and destroy the whole tumorous volume. The propulsion is made by applying magnetic field gradients (hence a magnetic force) generated by a Magnetic Stereotaxis System (MSS) which is a homemade device involving six supraconducting coils and a fluoroscopic imaging system. The VTF needs a 5 to 7 T/m magnetic field gradient in order to be able to move straight through thick brain tissue.
The researches from Tohoku University involve a magnet embedded inside a screw shaped body (between 1 and 2 mm in diameter and 8 to 15 mm in length). When applying a rotating magnetic field, the magnet and thus the screw tend to rotate and the system digs its path through tissues. The velocity of the screw shaped system is a function of the magnetic torque, rotational velocity of the field and pitch of the screw.